Dagger Notes fn + gh release create: remove softprops/action-gh-release

[haydenk]

Parent

Part of #14

What to build

• Add Notes(ctx, source *dagger.Directory, version, repo string, token *dagger.Secret) (string, error) to .dagger/main.go that renders the GitHub Release body — ## Images table from matrix.json + ## Usage block + GitHub-generated commit changelog appended via repos/{repo}/releases/generate-notes.
• Replace softprops/action-gh-release in release.yml with the idempotent gh release view → create / edit / upload pattern from haydenk/homestead.
• Add $GITHUB_STEP_SUMMARY block with a link to the published release.

Acceptance criteria

• dagger call notes --source=. --version=YYYYMMDD --repo=haydenk/docker-ansible-testing produces expected markdown locally
• release.yml uses gh release create (preinstalled gh, no third-party action)
• Re-running a release for an existing tag updates notes instead of failing
• Unit tests for Notes over a fixed matrix.json
• softprops/action-gh-release no longer referenced anywhere
• PAT-less

Blocked by

• Make release.yml callable: workflow_call + dry_run + RELEASE_TAG #16 (release.yml callable refactor)